This project was undertaken to study mutagenesis using monospecific antibodies against the sperm specific enzyme, lactate dehydrogenase-X. A monospecific antibody has been prepared from rabbit antiserum that reacts with rat sperm and not with mouse sperm. Using this antibody in a sandwich fluorescent antibody technique, point mutations in the mouse sperm were detected in mice treated with mutagenic drugs: procarbazine, mitomycin C, and ethylnitrosourea.